Which of the following is an example of a free rider problem?a$)$ Sally listens to public radio but never contributes to her local station.b$)$ Fred benefits when the National Guard arrives to help create a flood wall aroundhis town, but he is so poor that he pays no taxes.c$)$ Joe works with two other students on a group project for his intermediatemicroeconomics class. The grade is based on a group paper, but Joe goes to a movie instead of working on the paper because he knows that the other group members will write it without him.d$)$ All of the above are correct.